As aircraft development proceeds towards elimination of many hydromechanical controls in favor of all electric controls, weight reduction in non-electrical control elements becomes of great importance, especially in high performance aircraft which operate at elevated temperatures and altitudes. In the past, actuators for aircraft use have been proposed but were not completely satisfactory from the point of view of simplicity of construction, weight and reliability of operation. In addition, internal stops were used to prevent overstroking, and these stops determined the sizing of the other actuator parts.
Electromechanically actuated devices of a general nature are also well known. In conventional actuators, the ballnut typically remained stationary, and the ballscrew stroked back and forth by virtue of an arrangement of an actuating motor through a gear train. However, this arrangement was disadvantageous because the gear train multiplied the inertia torque of the motor by the square of the gear ratio. The inertia torque of stopping the motor very quickly (i.e., hitting the stops) was thus very high and required the actuator to be built with greater strength and weight.
The ball-screw linear actuator in U.S. Pat. No. 3,660,704 is a more compact linear actuator intended for spacecraft use. However, this device was not particularly lightweight. It achieved some compactness by mounting the motor on the ball nut but is not concerned with an arrangement which minimizes the need for a stiff, heavier housing. The housing requires an end plate or access cover at one end and a plurality of studs or bolts arranged in screw-threaded openings on mating flanges. A motor is arranged within the housing for driving a coupling in the form of a nut in a recirculating ball load screw assembly. A linear transducer extended into one end of the shaft continuously monitors the position of the shaft.
Actuators for a variety of other uses were also known. U.S. Pat. No. 3,161,074 describes an electromotive adjusting device primarily designed for an automatic opening or closing of mine ventilator doors. The device mounted a motor rotor directly on the ball nut and contained assorted springs, brakes, rotor locking and the like specific to the needs of mine door applications. It uses the rotary movement of a rotor to effect axial reciprocating movement of the rod. The nut is connected to and rotated with the rotor, and the screw then rotates to cause axial and rotary motion of the output of the ball screw.
U.S. Pat. No. 4,136,571 relates to a manure disposer installation in which an electric motor had a fixed stator and a rotor which was attached to a hollow cylinder on the interior of which is fixedly secured a threaded nut. The device drives a connecting rod to which manure shovers are attached to move to and from over a surface. A motor rotor is mounted on the nut rather than arranged to direct drive the ballscrew. The motor rotor has a hollow cylinder in which a spindle nut is fixed and is connected closely with a hollow cylinder. The spindle nut is threadably associated with a spindle so that the spindle is moved according to the rotating direction of the spindle nut in one of two axial directions of the hollow cylinder. This installation is not concerned with the problems of size and weight normally associated with actuators for use in environments where space and weight limitations are important such as, for example, in aircraft applications.
The prior art also is not concerned with how the actuator could lend itself to more modern manufacturing techniques such as composite manufacturing methods which allow the actuator to be lighter and yet used in more hostile environments, or how an actuator could be produced which avoided high inertia torque and thereby allowed for quick and accurate movement of the actuator.